I don’t know what this ex-employee did, whether they merely deleted files, performed a quick format, or wrote over the data. In addition to the possibility of wiping the entire drive, there are also secure delete utilities that wipe individual files, but It’s quite possible they aren’t sophisticated and the data is all still there if a person knows how to find it.
When I perform data destruction for clients, I usually use the drive’s built in secure erase feature because it’s the fastest way to wipe a drive and also ensures that spare sectors get erased as well.
You would think, but I’ve worked for some really bad managers in the past that could benefit from some self reflection if they were capable. While the ex-employee’s actions might have been inexcusable, they might not have been unprovoked.